BACKGROUND Endometriosis, a common condition among women of reproductive age and infertile women, occurs when the endometrium extends outside the uterus. When this endometrial tissue grows and sheds, symptoms will develop. The presentation varies depending on the site involved; however, cyclical pain is among its most common symptoms, along with bleeding and cramping. It is frequently observed in the ovaries and fallopian tubes; in contrast, the anal canal is rarely involved. Here, we report a very unusual presentation of the disease. CASE REPORT A 33-year-old woman with a history of episiotomy presented to the Emergency Department reporting perianal swelling in the previous year. The swelling was associated with intermittent pain and difficulty passing stool. She reported no fever. On examination, there was a 3×4 cm palpable tender perianal mass extending to the anal sphincter at the 11 o'clock position. Bedside ultrasound revealed a mass. Magnetic resonance imaging showed a hemorrhagic 3×4 cm mass in the right perianal region pressing on and indenting the right aspect of the distal external sphincter. The mass was excised completely with local perianal incision over the mass at 11'o clock. Surgical pathology revealed an isolated endometrioma in the perianal area. CONCLUSIONS Isolated perianal endometrioma is a rare disease, with only 21 published cases. Its diagnosis is difficult to establish, and a wide range of tests is often needed. Laparoscopic or surgical intervention may be required in cases of rectal endometriosis for an accurate diagnosis. Careful history taking and examination along with a high index of suspicion are necessary to diagnose perianal endometrioma.